1. Field of Invention
The present invention relates to a demodulator, and particularly to a frequency shift keying demodulator robust for frequency offset.
2. Description of the Related Art
To transmit signals with a wireless communication system, the information signals to be transmitted must be modulated into a sine wave, followed by sending out the modulated signals. One of the normal modulation modes is called frequency modulation (FM), which is widely used in FM radio and other wireless equipments, such as beepers, mobile phones and wireless phones. Among various FM techniques, there is a so-called “frequency shift keying modulation” (FSK modulation) method, by which a digital data is frequency-modulated with two frequencies f1 and f0 and the modulated signal is sent out. Once the signal modulated with frequency shift keying is transmitted and received by a receiver, the received signal would be restored to its original digital data through demodulation processing.
FIG. 1 is a block diagram of a conventional frequency shift keying receiver. A frequency shift keying receiver 100 includes a receiving antenna 110, a low-noise amplifier (LNA) 120, a frequency mixer 130, a low-pass filter 140, an analog-to-digital converter (ADC) 150 and a demodulator 160. The receiving antenna 110 receives RF signal (radio frequency signal) transmitted by a transmitter. The received signal is then amplified by the LNA 120. Further, the amplified signal is frequency-mixed with a local oscillating signal fc by the frequency mixer 130, and then produces two part of signals, the high frequency term signal and the low frequency term signal. Then the high frequency term signal is removed by the low-pass filter 140. Furthermore, the signal is sampled by the analog-to-digital converter (ADC) 150 and a digital non-coherent frequency shift key signal (DNFSK signal) is obtained. The DNFSK signal containing modulated data at f1 and f0 is then demodulated by the demodulator 160 to finally get the digital signal transmitted by a transmitter.
FIG. 2 is a block diagram of a conventional frequency shift keying demodulator robust for frequency offset. The conventional frequency shift keying demodulator includes a frequency synthesizer 210, a first frequency mixer 211, a first low-pass filter 212, a second frequency mixer 213, a phase difference generator 214, a second low-pass filter 215 and a decider 216.
First, the first frequency mixer 211 receives a digital non-coherent frequency shift keying signal DNFSK and a frequency synthesizer output signal, for example, a 50 KHz cosine signal, generated by the frequency synthesizer 210. The two received signals are frequency-mixed. Next, the first mixer output is filtered by the first low-pass filter 212 and a baseband signal BS is obtained. The baseband signal BS is the input to the phase difference generator 214 and the second frequency mixer 213. Wherein, the phase difference generator 214 would produce a baseband signal BSQ which has π/4 phase-shift with respect to the baseband signal BS and send it to the input of second frequency mixer 213. After that, the second frequency mixer 213 would produce another high frequency term signal and low frequency term signal. Finally, the second low-pass filter 215 would filter out the high frequency term signal and left low frequency term signal. The low frequency term signal is the demodulated output. The above-described process can be expressed by the following formulas. First, it is assumed that the baseband signal received by the second frequency mixer 213 isBS=cos(ωbt)BSQ=cos(ωb(t−τ))for transmitting logic 1,ωb=2π(fb+f1)for transmitting logic 0,ωb=2π(fb−f1)In addition, 2πfbτ=π/2After frequency-mixing by the second frequency mixer 213,
                    MIX        =                              cos            ⁡                          (                                                ω                  b                                ⁢                t                            )                                *                      cos            ⁡                          (                                                ω                  b                                ⁡                                  (                                      t                    -                    τ                                    )                                            )                                                              =                                            1              2                        ⁢                          cos              ⁡                              (                                                      2                    ⁢                                          ω                      b                                        ⁢                    t                                    -                                                            ω                      b                                        ⁢                    τ                                                  )                                              +                                    1              2                        ⁢                          cos              ⁡                              (                                                      ω                    b                                    ⁢                  τ                                )                                                        After being filtered by the second low-pass filter 215, the following is obtained:
            1      2        ⁢          cos      ⁡              (                              ω            b                    ⁢          τ                )              =                    1        2            ⁢              cos        ⁡                  (                      2            ⁢                          π              ⁡                              (                                                      f                    b                                    ±                                      f                    1                                                  )                                      ⁢            τ                    )                      =                  ∓                  1          2                    ⁢              sin        ⁡                  (                      2            ⁢            π            ⁢                                                  ⁢                          f              1                        ⁢            τ                    )                    The decider 216 receives the signal output from the second low-pass filter 215; and according to positive/negative polarity of the sine wave a digital signal is demodulated.
FIG. 3 is a diagram showing digital signal waveforms of the transmitted data and the demodulated data, respectively. Referring to FIG. 3, the waveform 201 is a digital-signal-waveform of preamble transmitted by FSKT (frequency shift keying transmitter), the waveform 202 is a digital-signal-waveform of preamble demodulated by FSKR (frequency shift keying receiver) with little frequency offset and the waveform 203 is a demodulated digital-signal-waveform of preamble with large frequency offset. It can be seen from FIG. 3, if there is no frequency offset occurs with a general FSKR, the duty cycle of the demodulated preamble digital signal would be close to 50%; while for a general FSKR influenced by a frequency offset, the duty cycle of the demodulated preamble digital signal would be departed from 50%, which would significantly increase the possibility of system errors.
To eliminate the above-described problem, a conventional solution is that a frequency-adjustable frequency synthesizer 210, for example, a variable resistor (or variable capacitor, variable inductor and so on) is added in the system and the variable resistor (or variable capacitor, variable inductor and so on) is manually adjusted on production lines before shipment, so that the duty cycle of the demodulated preamble digital signal is close to 50%. The solution, however, is wasteful of manpower and cost that the production efficiency would drop.